An analytical chemist has determined by measurements that there are moles of magnesium in a sample of talc. how many moles of silicon are in the sample

The chemical formula for talc is:
Mg₃(SiO₂)₅(OH)₂

The formula shows us that there are:
3 moles of magnesium
5 moles of silicon
12 moles of oxygen
2 moles of hydrogen
For every mole of talc

Using this, we can determine the ratio between magnesium and silicon. This is:
3 moles Mg : 5 moles Si

Given a number of moles of magnesium, we may simply multiply by 5/3 to determine the number of moles of silicon present.